The present invention generally relates to feeders and in one preferred embodiment a timed feeder and method of feeding articles such as sheets towards one or more stations where an operation is performed on the article. For example, in the corrugated board art, a timed feeder is used to feed corrugated boards to a box finishing machine where the boards are slit, slotted and/or scored, and printed. It is essential that the boards be fed in synchronism or in “register” with operations performed on the board downstream. Therefore the time it takes for each fed board to reach the same location downstream is always the same. That is to say that for a given process each board is fed at the same time cycle or interval, and the distance between the leading ends of successive boards is always the same. Typically the boards are first fed to nip rolls which then feed the boards downstream to a printer after which the boards are conveyed to a slitting, slotting or scoring die. Various examples of timed corrugated board feeders may be found in U.S. Pat. Nos. 4,045,015; 4,494,745; 4,632,378; 4,681,311; 4,889,331 and 5,184,811.
With timed feeders such as those identified above, the distance between successive boards measured between their leading ends is constant, and this distance is called the “repeat length” in the art. Where the finishing machine includes a rotatable print cylinder, the circumferential length of the print cylinder is equal to the repeat length of the feeder. In the box finishing art, timed feeders are used to feed boards of different sizes, but with the same repeat length. This is inefficient because the space between the boards increases when different boards of shorter length are processed. This slows down production rate and also can cause loss of vacuum in machines which utilize vacuum in conveying the boards.
In the art of “one pass” digital printing where the printing is completed in one pass of the sheet being printed, the gap or space between the sheets being printed should be at a minimum if not nil to avoid air flow between the gap which can adversely affect the printing.
In industry there is a need for a timed feeder which is practical or feasible to use and at the same time allows the repeat length or time cycle of the feeder between successive sheets, to be easily adjusted to accommodate articles, such as sheets, of different lengths. In an attempt to provide such a feeder, one may envision the use of programmable servo motors to directly drive the feeding elements of a feeder. This would allow the speed of the feeding elements, and consequently the time cycle, to be changed as desired in order to arrive at a suitable time cycle or repeat length depending upon the length of an article being fed. However due to the relatively high loads from the inertia of the drive transmission system and the sheet being fed as well as from the vacuum forces imposed on the belts and sheets, this approach is not believed to be satisfactory because it would require very large and cumbersome servo motors and space to house them while also being difficult to design, all of which renders the proposition impractical or too expensive if not unfeasible.
In an attempt to reduce or close the gap between the fed articles, one might also envision driving downstream nip rolls with servo motors. However this would still be unsatisfactory because the drive of the nip rolls would conflict with the drive elements downstream of the nip rolls. In addition it would increase the expense and complicate the nip roll drive system.
Although rotating stream feeders are capable of feeding sheets with relatively high speed and small gaps between sheets, they are not suitable for timed feeding because the sheets are subject to slippages and the size of the gaps between the sheets are not consistent such that the sheets cannot be consistently fed with register or synchronism with downstream operations to be performed on the sheets.